EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-PH-EP-2013-150 LHCb-PAPER-2013-035 21 August 2013
Observation of the decay B
s
0
→ D
0
φ
The LHCb collaboration†
Abstract
First observation of the decay Bs0→ D0φ is reported using pp collision data, corre-sponding to an integrated luminosity of 1.0 fb−1, collected by the LHCb experiment at a centre-of-mass energy of 7 TeV. The significance of the signal is 6.5 standard devi-ations. The branching fraction is measured relative to that of the decay Bs0→ D0K∗0 to be B(B0 s→ D0φ) B(B0 s→ D0K∗0) = 0.069 ± 0.013 (stat) ± 0.007 (syst).
The first measurement of the ratio of branching fractions for the decays Bs0→ D0K∗0 and B0→ D0K∗0 is found to be
B(B0
s→ D0K∗0)
B(B0→ D0K∗0) = 7.8 ± 0.7 (stat) ± 0.3 (syst) ± 0.6 (fs/fd), where the last uncertainty is due to the ratio of the B0
s and B0 fragmentation fractions.
Submitted to Phys. Lett. B
c
CERN on behalf of the LHCb collaboration, license CC-BY-3.0.
†Authors are listed on the following pages.
LHCb collaboration
R. Aaij40, B. Adeva36, M. Adinolfi45, C. Adrover6, A. Affolder51, Z. Ajaltouni5, J. Albrecht9, F. Alessio37, M. Alexander50, S. Ali40, G. Alkhazov29, P. Alvarez Cartelle36, A.A. Alves Jr24,37, S. Amato2, S. Amerio21, Y. Amhis7, L. Anderlini17,f, J. Anderson39, R. Andreassen56,
J.E. Andrews57, R.B. Appleby53, O. Aquines Gutierrez10, F. Archilli18, A. Artamonov34, M. Artuso58, E. Aslanides6, G. Auriemma24,m, M. Baalouch5, S. Bachmann11, J.J. Back47, C. Baesso59, V. Balagura30, W. Baldini16, R.J. Barlow53, C. Barschel37, S. Barsuk7,
W. Barter46, Th. Bauer40, A. Bay38, J. Beddow50, F. Bedeschi22, I. Bediaga1, S. Belogurov30, K. Belous34, I. Belyaev30, E. Ben-Haim8, G. Bencivenni18, S. Benson49, J. Benton45,
A. Berezhnoy31, R. Bernet39, M.-O. Bettler46, M. van Beuzekom40, A. Bien11, S. Bifani44, T. Bird53, A. Bizzeti17,h, P.M. Bjørnstad53, T. Blake37, F. Blanc38, J. Blouw11, S. Blusk58, V. Bocci24, A. Bondar33, N. Bondar29, W. Bonivento15, S. Borghi53, A. Borgia58,
T.J.V. Bowcock51, E. Bowen39, C. Bozzi16, T. Brambach9, J. van den Brand41, J. Bressieux38, D. Brett53, M. Britsch10, T. Britton58, N.H. Brook45, H. Brown51, I. Burducea28, A. Bursche39, G. Busetto21,q, J. Buytaert37, S. Cadeddu15, O. Callot7, M. Calvi20,j, M. Calvo Gomez35,n, A. Camboni35, P. Campana18,37, D. Campora Perez37, A. Carbone14,c, G. Carboni23,k, R. Cardinale19,i, A. Cardini15, H. Carranza-Mejia49, L. Carson52, K. Carvalho Akiba2, G. Casse51, L. Castillo Garcia37, M. Cattaneo37, Ch. Cauet9, R. Cenci57, M. Charles54, Ph. Charpentier37, P. Chen3,38, N. Chiapolini39, M. Chrzaszcz25, K. Ciba37, X. Cid Vidal37, G. Ciezarek52, P.E.L. Clarke49, M. Clemencic37, H.V. Cliff46, J. Closier37, C. Coca28, V. Coco40, J. Cogan6, E. Cogneras5, P. Collins37, A. Comerma-Montells35, A. Contu15,37, A. Cook45, M. Coombes45, S. Coquereau8, G. Corti37, B. Couturier37, G.A. Cowan49, D.C. Craik47, S. Cunliffe52, R. Currie49, C. D’Ambrosio37, P. David8, P.N.Y. David40, A. Davis56, I. De Bonis4, K. De Bruyn40, S. De Capua53, M. De Cian11, J.M. De Miranda1, L. De Paula2, W. De Silva56, P. De Simone18, D. Decamp4, M. Deckenhoff9, L. Del Buono8, N. D´el´eage4, D. Derkach54, O. Deschamps5, F. Dettori41, A. Di Canto11, H. Dijkstra37, M. Dogaru28, S. Donleavy51, F. Dordei11, A. Dosil Su´arez36, D. Dossett47, A. Dovbnya42, F. Dupertuis38, P. Durante37, R. Dzhelyadin34, A. Dziurda25, A. Dzyuba29, S. Easo48, U. Egede52, V. Egorychev30,
S. Eidelman33, D. van Eijk40, S. Eisenhardt49, U. Eitschberger9, R. Ekelhof9, L. Eklund50,37, I. El Rifai5, Ch. Elsasser39, A. Falabella14,e, C. F¨arber11, G. Fardell49, C. Farinelli40, S. Farry51, D. Ferguson49, V. Fernandez Albor36, F. Ferreira Rodrigues1, M. Ferro-Luzzi37, S. Filippov32, M. Fiore16, C. Fitzpatrick37, M. Fontana10, F. Fontanelli19,i, R. Forty37, O. Francisco2, M. Frank37, C. Frei37, M. Frosini17,f, S. Furcas20, E. Furfaro23,k, A. Gallas Torreira36,
D. Galli14,c, M. Gandelman2, P. Gandini58, Y. Gao3, J. Garofoli58, P. Garosi53, J. Garra Tico46, L. Garrido35, C. Gaspar37, R. Gauld54, E. Gersabeck11, M. Gersabeck53, T. Gershon47,37, Ph. Ghez4, V. Gibson46, L. Giubega28, V.V. Gligorov37, C. G¨obel59, D. Golubkov30, A. Golutvin52,30,37, A. Gomes2, P. Gorbounov30,37, H. Gordon37, C. Gotti20,
M. Grabalosa G´andara5, R. Graciani Diaz35, L.A. Granado Cardoso37, E. Graug´es35,
G. Graziani17, A. Grecu28, E. Greening54, S. Gregson46, P. Griffith44, O. Gr¨unberg60, B. Gui58, E. Gushchin32, Yu. Guz34,37, T. Gys37, C. Hadjivasiliou58, G. Haefeli38, C. Haen37,
S.C. Haines46, S. Hall52, B. Hamilton57, T. Hampson45, S. Hansmann-Menzemer11, N. Harnew54, S.T. Harnew45, J. Harrison53, T. Hartmann60, J. He37, T. Head37, V. Heijne40, K. Hennessy51, P. Henrard5, J.A. Hernando Morata36, E. van Herwijnen37, M. Hess60, A. Hicheur1, E. Hicks51, D. Hill54, M. Hoballah5, C. Hombach53, P. Hopchev4, W. Hulsbergen40, P. Hunt54, T. Huse51, N. Hussain54, D. Hutchcroft51, D. Hynds50, V. Iakovenko43, M. Idzik26, P. Ilten12,
R. Jacobsson37, A. Jaeger11, E. Jans40, P. Jaton38, A. Jawahery57, F. Jing3, M. John54, D. Johnson54, C.R. Jones46, C. Joram37, B. Jost37, M. Kaballo9, S. Kandybei42, W. Kanso6, M. Karacson37, T.M. Karbach37, I.R. Kenyon44, T. Ketel41, A. Keune38, B. Khanji20, O. Kochebina7, I. Komarov38, R.F. Koopman41, P. Koppenburg40, M. Korolev31, A. Kozlinskiy40, L. Kravchuk32, K. Kreplin11, M. Kreps47, G. Krocker11, P. Krokovny33, F. Kruse9, M. Kucharczyk20,25,j, V. Kudryavtsev33, T. Kvaratskheliya30,37, V.N. La Thi38, D. Lacarrere37, G. Lafferty53, A. Lai15, D. Lambert49, R.W. Lambert41, E. Lanciotti37,
G. Lanfranchi18, C. Langenbruch37, T. Latham47, C. Lazzeroni44, R. Le Gac6, J. van Leerdam40, J.-P. Lees4, R. Lef`evre5, A. Leflat31, J. Lefran¸cois7, S. Leo22, O. Leroy6, T. Lesiak25,
B. Leverington11, Y. Li3, L. Li Gioi5, M. Liles51, R. Lindner37, C. Linn11, B. Liu3, G. Liu37, S. Lohn37, I. Longstaff50, J.H. Lopes2, N. Lopez-March38, H. Lu3, D. Lucchesi21,q, J. Luisier38, H. Luo49, F. Machefert7, I.V. Machikhiliyan4,30, F. Maciuc28, O. Maev29,37, S. Malde54,
G. Manca15,d, G. Mancinelli6, J. Maratas5, U. Marconi14, P. Marino22,s, R. M¨arki38, J. Marks11, G. Martellotti24, A. Martens8, A. Mart´ın S´anchez7, M. Martinelli40, D. Martinez Santos41, D. Martins Tostes2, A. Martynov31, A. Massafferri1, R. Matev37, Z. Mathe37, C. Matteuzzi20, E. Maurice6, A. Mazurov16,32,37,e, J. McCarthy44, A. McNab53, R. McNulty12, B. McSkelly51, B. Meadows56,54, F. Meier9, M. Meissner11, M. Merk40, D.A. Milanes8, M.-N. Minard4, J. Molina Rodriguez59, S. Monteil5, D. Moran53, P. Morawski25, A. Mord`a6, M.J. Morello22,s, R. Mountain58, I. Mous40, F. Muheim49, K. M¨uller39, R. Muresan28, B. Muryn26, B. Muster38, P. Naik45, T. Nakada38, R. Nandakumar48, I. Nasteva1, M. Needham49, S. Neubert37,
N. Neufeld37, A.D. Nguyen38, T.D. Nguyen38, C. Nguyen-Mau38,o, M. Nicol7, V. Niess5,
R. Niet9, N. Nikitin31, T. Nikodem11, A. Nomerotski54, A. Novoselov34, A. Oblakowska-Mucha26, V. Obraztsov34, S. Oggero40, S. Ogilvy50, O. Okhrimenko43, R. Oldeman15,d, M. Orlandea28, J.M. Otalora Goicochea2, P. Owen52, A. Oyanguren35, B.K. Pal58, A. Palano13,b,
T. Palczewski27, M. Palutan18, J. Panman37, A. Papanestis48, M. Pappagallo50, C. Parkes53, C.J. Parkinson52, G. Passaleva17, G.D. Patel51, M. Patel52, G.N. Patrick48, C. Patrignani19,i, C. Pavel-Nicorescu28, A. Pazos Alvarez36, A. Pellegrino40, G. Penso24,l, M. Pepe Altarelli37, S. Perazzini14,c, E. Perez Trigo36, A. P´erez-Calero Yzquierdo35, P. Perret5, M. Perrin-Terrin6, L. Pescatore44, E. Pesen61, K. Petridis52, A. Petrolini19,i, A. Phan58, E. Picatoste Olloqui35, B. Pietrzyk4, T. Pilaˇr47, D. Pinci24, S. Playfer49, M. Plo Casasus36, F. Polci8, G. Polok25, A. Poluektov47,33, E. Polycarpo2, A. Popov34, D. Popov10, B. Popovici28, C. Potterat35, A. Powell54, J. Prisciandaro38, A. Pritchard51, C. Prouve7, V. Pugatch43, A. Puig Navarro38, G. Punzi22,r, W. Qian4, J.H. Rademacker45, B. Rakotomiaramanana38, M.S. Rangel2,
I. Raniuk42, N. Rauschmayr37, G. Raven41, S. Redford54, M.M. Reid47, A.C. dos Reis1,
S. Ricciardi48, A. Richards52, K. Rinnert51, V. Rives Molina35, D.A. Roa Romero5, P. Robbe7, D.A. Roberts57, E. Rodrigues53, P. Rodriguez Perez36, S. Roiser37, V. Romanovsky34,
A. Romero Vidal36, J. Rouvinet38, T. Ruf37, F. Ruffini22, H. Ruiz35, P. Ruiz Valls35, G. Sabatino24,k, J.J. Saborido Silva36, N. Sagidova29, P. Sail50, B. Saitta15,d,
V. Salustino Guimaraes2, B. Sanmartin Sedes36, M. Sannino19,i, R. Santacesaria24,
C. Santamarina Rios36, E. Santovetti23,k, M. Sapunov6, A. Sarti18,l, C. Satriano24,m, A. Satta23, M. Savrie16,e, D. Savrina30,31, P. Schaack52, M. Schiller41, H. Schindler37, M. Schlupp9,
M. Schmelling10, B. Schmidt37, O. Schneider38, A. Schopper37, M.-H. Schune7, R. Schwemmer37, B. Sciascia18, A. Sciubba24, M. Seco36, A. Semennikov30, K. Senderowska26, I. Sepp52,
N. Serra39, J. Serrano6, P. Seyfert11, M. Shapkin34, I. Shapoval16,42, P. Shatalov30,
Y. Shcheglov29, T. Shears51,37, L. Shekhtman33, O. Shevchenko42, V. Shevchenko30, A. Shires9, R. Silva Coutinho47, M. Sirendi46, N. Skidmore45, T. Skwarnicki58, N.A. Smith51, E. Smith54,48,
J. Smith46, M. Smith53, M.D. Sokoloff56, F.J.P. Soler50, F. Soomro18, D. Souza45, B. Souza De Paula2, B. Spaan9, A. Sparkes49, P. Spradlin50, F. Stagni37, S. Stahl11, O. Steinkamp39, S. Stevenson54, S. Stoica28, S. Stone58, B. Storaci39, M. Straticiuc28,
U. Straumann39, V.K. Subbiah37, L. Sun56, S. Swientek9, V. Syropoulos41, M. Szczekowski27, P. Szczypka38,37, T. Szumlak26, S. T’Jampens4, M. Teklishyn7, E. Teodorescu28, F. Teubert37, C. Thomas54, E. Thomas37, J. van Tilburg11, V. Tisserand4, M. Tobin38, S. Tolk41, D. Tonelli37, S. Topp-Joergensen54, N. Torr54, E. Tournefier4,52, S. Tourneur38, M.T. Tran38, M. Tresch39, A. Tsaregorodtsev6, P. Tsopelas40, N. Tuning40, M. Ubeda Garcia37, A. Ukleja27, D. Urner53, A. Ustyuzhanin52,p, U. Uwer11, V. Vagnoni14, G. Valenti14, A. Vallier7, M. Van Dijk45,
R. Vazquez Gomez18, P. Vazquez Regueiro36, C. V´azquez Sierra36, S. Vecchi16, J.J. Velthuis45, M. Veltri17,g, G. Veneziano38, M. Vesterinen37, B. Viaud7, D. Vieira2, X. Vilasis-Cardona35,n, A. Vollhardt39, D. Volyanskyy10, D. Voong45, A. Vorobyev29, V. Vorobyev33, C. Voß60,
H. Voss10, R. Waldi60, C. Wallace47, R. Wallace12, S. Wandernoth11, J. Wang58, D.R. Ward46, N.K. Watson44, A.D. Webber53, D. Websdale52, M. Whitehead47, J. Wicht37, J. Wiechczynski25, D. Wiedner11, L. Wiggers40, G. Wilkinson54, M.P. Williams47,48, M. Williams55, F.F. Wilson48, J. Wimberley57, J. Wishahi9, W. Wislicki27, M. Witek25, S.A. Wotton46, S. Wright46, S. Wu3, K. Wyllie37, Y. Xie49,37, Z. Xing58, Z. Yang3, R. Young49, X. Yuan3, O. Yushchenko34,
M. Zangoli14, M. Zavertyaev10,a, F. Zhang3, L. Zhang58, W.C. Zhang12, Y. Zhang3, A. Zhelezov11, A. Zhokhov30, L. Zhong3, A. Zvyagin37.
1Centro Brasileiro de Pesquisas F´ısicas (CBPF), Rio de Janeiro, Brazil 2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil 3Center for High Energy Physics, Tsinghua University, Beijing, China 4LAPP, Universit´e de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France
5Clermont Universit´e, Universit´e Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France 6CPPM, Aix-Marseille Universit´e, CNRS/IN2P3, Marseille, France
7LAL, Universit´e Paris-Sud, CNRS/IN2P3, Orsay, France
8LPNHE, Universit´e Pierre et Marie Curie, Universit´e Paris Diderot, CNRS/IN2P3, Paris, France 9Fakult¨at Physik, Technische Universit¨at Dortmund, Dortmund, Germany
10Max-Planck-Institut f¨ur Kernphysik (MPIK), Heidelberg, Germany
11Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany 12School of Physics, University College Dublin, Dublin, Ireland
13Sezione INFN di Bari, Bari, Italy 14Sezione INFN di Bologna, Bologna, Italy 15Sezione INFN di Cagliari, Cagliari, Italy 16Sezione INFN di Ferrara, Ferrara, Italy 17Sezione INFN di Firenze, Firenze, Italy
18Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy 19Sezione INFN di Genova, Genova, Italy
20Sezione INFN di Milano Bicocca, Milano, Italy 21Sezione INFN di Padova, Padova, Italy 22Sezione INFN di Pisa, Pisa, Italy
23Sezione INFN di Roma Tor Vergata, Roma, Italy 24Sezione INFN di Roma La Sapienza, Roma, Italy
25Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krak´ow, Poland 26AGH - University of Science and Technology, Faculty of Physics and Applied Computer Science,
Krak´ow, Poland
27National Center for Nuclear Research (NCBJ), Warsaw, Poland
28Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania 29Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia
30Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia
31Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia
32Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia 33Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia 34Institute for High Energy Physics (IHEP), Protvino, Russia
35Universitat de Barcelona, Barcelona, Spain
36Universidad de Santiago de Compostela, Santiago de Compostela, Spain 37European Organization for Nuclear Research (CERN), Geneva, Switzerland 38Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne, Switzerland 39Physik-Institut, Universit¨at Z¨urich, Z¨urich, Switzerland
40Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands
41Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The
Netherlands
42NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine
43Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine 44University of Birmingham, Birmingham, United Kingdom
45H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 46Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 47Department of Physics, University of Warwick, Coventry, United Kingdom 48STFC Rutherford Appleton Laboratory, Didcot, United Kingdom
49School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 50School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom 51Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom 52Imperial College London, London, United Kingdom
53School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom 54Department of Physics, University of Oxford, Oxford, United Kingdom
55Massachusetts Institute of Technology, Cambridge, MA, United States 56University of Cincinnati, Cincinnati, OH, United States
57University of Maryland, College Park, MD, United States 58Syracuse University, Syracuse, NY, United States
59Pontif´ıcia Universidade Cat´olica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to 2 60Institut f¨ur Physik, Universit¨at Rostock, Rostock, Germany, associated to11
61Celal Bayar University, Manisa, Turkey, associated to37
aP.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia bUniversit`a di Bari, Bari, Italy
cUniversit`a di Bologna, Bologna, Italy dUniversit`a di Cagliari, Cagliari, Italy eUniversit`a di Ferrara, Ferrara, Italy fUniversit`a di Firenze, Firenze, Italy gUniversit`a di Urbino, Urbino, Italy
hUniversit`a di Modena e Reggio Emilia, Modena, Italy iUniversit`a di Genova, Genova, Italy
jUniversit`a di Milano Bicocca, Milano, Italy kUniversit`a di Roma Tor Vergata, Roma, Italy lUniversit`a di Roma La Sapienza, Roma, Italy mUniversit`a della Basilicata, Potenza, Italy
nLIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain oHanoi University of Science, Hanoi, Viet Nam
pInstitute of Physics and Technology, Moscow, Russia qUniversit`a di Padova, Padova, Italy
1
Introduction
Measurements of the decay1 B0
s→ D0φ are of particular interest because they provide
information that can be used to determine the CKM angles γ ≡ arg[−VudVub∗/(VcdVcb∗)]
and βs ≡ arg[−VtsVtb∗/(VcsVcb∗)] without theoretical uncertainties [1]. Knowledge of these
CP -violating phases is crucial to search for new sources of CP violation and unravel subtle effects of physics beyond the Standard Model, which may appear in flavour-changing interactions. Their precise measurements are among the most important goals of flavour physics experiments.
To date, the angle γ is the least well-determined angle of the Unitarity Triangle with an uncertainty of about 10◦ [2–4]. The current precision is dominated by measurements of time-integrated B+ → DK+ decay rates, where D indicates a superposition of D0 and D0
decays to a common final state. In these decays, sensitivity to γ arises from direct CP violation in the interference between the b → c¯us and b → u¯cs tree-level amplitudes. As there are no loop contributions to the decay amplitudes, no theoretical uncertainties arise. The main limitation is due to the size of the data samples collected by the experiments. To improve on the precision, it is important to perform additional measurements from other channels with small theoretical uncertainties.
The large production cross-section of Bs0 mesons in pp collisions at the LHC opens new possibilities for measuring both γ and βs. For example, the decay Bs0 → D
± s K
∓ is
sensitive to γ + 2βs through measurements of time-dependent decay rates [5, 6]; although
the determination of γ from this mode requires an independent measurement of the mixing phase βs.
The decay B0
s→ D0φ, first proposed in 1991 by Gronau and London for measuring γ [7],
can also probe βs via measurements of time-dependent decay rates. Nandi and London
have shown [1] that both γ and βs can be determined without theoretical uncertainties
and ambiguities, using the known sign of ∆Γs, the decay-width difference between the two
Bs0 mass eigenstates [8].
An alternative method to measure γ using B0
s → Dφ decays was proposed in Refs. [9,10],
where it was shown that γ can be determined from time-integrated decay rates, in a similar way as from B+→ DK+ decays, even if B0
s → Dφ is not a self-tagged decay mode. The
only requirement for the determination is that a sufficient number of different D final states are included in the measurement. The time-integrated method does not require flavour-tagging, and hence makes optimal use of the statistical power of the large bb production at LHC. An estimation of the sensitivity with this method shows that the mode B0
s → Dφ has the potential to make a significant impact on the determination of γ
at LHCb [11].
The observation of the Bs0→ D0φ decay and the measurement of its branching fraction,
described in this Letter, are the first steps towards a programme of CP violation studies with this channel. The branching fraction is measured relative to the topologically similar decay Bs0→ D0K∗0
, that was previously observed by LHCb [12]. In addition, the first measurement of the branching fraction of the B0
s → D0K
∗0 decay relative to the
0 s B W+ b c s s u s 0 D φ (a) 0 s B W+ b u s s c s 0 D φ (b) 0 B + W b c d d u s 0 D *0 K (c) 0 s B W+ b c s s u d 0 D *0 K (d)
Figure 1: Feynman diagrams for the following decays: (a) Bs0→ D0φ; (b) B0
s → D0φ; (c) B0→ D0K∗0; and (d) B0
s→ D0K∗0. The Bs0→ D0φ and Bs0 → D0φ decay amplitudes interfere when D0 and D0 decay to the same final state.
B0→ D0K∗0 decay is reported and used to improve on the knowledge of the branching
fraction of the Bs0→ D0K∗0decay. The Feynman diagrams corresponding to the B0
s→ D0φ
and B0
s → D0φ decay amplitudes are shown in Fig. 1. The Feynman diagrams for the
leading b → c amplitudes in B0
s→ D0K∗0 and B0→ D0K∗0decays are also shown in Fig. 1.
Since only D0 → K−π+ decays are considered in this study, all of the measured quantities
for the B0
s → D0φ, Bs0 → D0K
∗0, and B0 → D0K∗0 channels include contributions
from the B0
s → D0φ, Bs0 → D0K∗0, and B0 → D0K∗0 modes, respectively, through the
doubly-Cabibbo-suppressed decay D0 → K+π−.
2
Event selection
The study reported here is based on pp collision data, corresponding to an integrated luminosity of 1.0 fb−1, collected by the LHCb experiment at a centre-of-mass energy of 7 TeV. The LHCb detector [13] is a single-arm forward spectrometer covering the pseudorapidity range 2 < η < 5, designed for the study of particles containing b or c quarks. The detector includes a high-precision tracking system consisting of a silicon-strip vertex detector surrounding the pp interaction region, a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4 Tm, and three stations of silicon-strip detectors and straw drift tubes placed downstream. The combined tracking system provides a momentum (p) measurement with relative uncertainty that varies from 0.4% at 5 GeV/c to 0.6% at 100 GeV/c, and impact parameter (IP) resolution of 20 µm for tracks with large transverse momentum (pT). Charged hadrons are identified using
two ring-imaging Cherenkov detectors [14]. Photon, electron and hadron candidates are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system composed of alternating layers of iron and multiwire proportional chambers [15].
The trigger [16] consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage, which applies a full event reconstruction. Simulated signal samples and data control channels are used to optimise the selection criteria. In the simulation, pp collisions are generated using Pythia 6.4 [17] with a specific LHCb configuration [18]. Decays of hadrons are described by EvtGen [19], in which final state radiation is generated using Photos [20]. The interaction of the generated particles with the detector and its response are implemented using the Geant4 toolkit [21] as described in Ref. [22].
Selected events fulfill one of two hardware trigger requirements: either a particle from the signal decay deposits enough energy in the calorimeter system, or one of the particles in the event, not originating from the signal decay, fulfils any of the trigger requirements (e.g., events triggered by one or more particles coming from the decay of the other B meson in the pp → b¯bX event). The software trigger requires a two-, three- or four-track secondary vertex with a large scalar sum of the tracks pT and significant displacement
from the associated primary pp interaction vertex (PV). At least one track should have pT > 1.7 GeV/c and a value of χ2IP > 16, where χ2IP is defined as the difference between
the χ2 of the PV reconstructed with and without the considered particle. A multivariate
algorithm identifies secondary vertices consistent with the decay of a b hadron.
Reconstructed tracks are selected with criteria on their p, pT, track χ2 per degree of
freedom, χ2
IP and particle identification (PID). Tracks identified as muons are discarded.
The D0 mesons are reconstructed in the decay mode D0 → K−π+. Particle
identifica-tion criteria used to select the daughters require the difference between the log-likelihoods of the kaon and pion hypotheses (∆LLKπ) to be larger than 0 for the kaon and smaller
than 4 for the pion. The D0 meson χ2
IP is required to be larger than 2 to separate mesons
originating from a B decay and those produced at the PV. In addition, for the D0K∗0 (D0K∗0) final states, the charm meson flight distance with respect to the B0
(s) vertex is
required to be larger than 0 with a significance of at least 2 standard deviations in order to suppress background from B(s)0 decays without an intermediate charm meson, such as the mode B0 → K−π+K∗0. There is no corresponding requirement in the D0φ final
state, since the charmless background is negligible. The D0 candidates with invariant
mass within ±20 MeV/c2 of the known mass [23] are retained. The φ mesons are reconstructed in the mode φ → K+K−. The p
Tof the kaon daughters
is required to be larger than 350 MeV/c and the ∆LLKπ of both daughters to be larger
than 3. Candidates are retained if their invariant mass is within ±10 MeV/c2 of the known φ mass [23].
The K∗0mesons are reconstructed in the mode K∗0→ K+π−. The p
Tof the kaon (pion)
is required to be larger than 350 (250) MeV/c. In addition, to reduce the cross-feed from B0 → D0ρ0 and B0 → D0K+K− decays, the ∆LL
Kπ of the kaon must be larger than 3
and that of the pion smaller than 3. Possible background from protons in the kaon sample, for example from the decay Λ0b → D0pπ−, is suppressed by selecting kaon candidates with
a difference between the log-likelihoods of proton and kaon hypotheses, ∆LLpK, smaller
than 10. Candidate K∗0 mesons with invariant mass within ±50 MeV/c2 of the known
Neutral B meson candidates are formed from D0 and φ (or K∗0) candidates, which are fitted to a common vertex with the D0 constrained to its known mass. In order
to reduce contributions from non-resonant decays, B0
(s) → D0K+K −, B0
s → D0K −π+,
and B0 → D0K+π− [24, 25], the absolute value of the cosine of the vector-daughter
helicity angle (cos θh) is required to be larger than 0.4. This angle is defined between
the momentum direction of the K+ daughter in the φ (K∗0) frame, and the vector meson
direction in the B rest frame. Backgrounds from B(s)0 → D(s)∓ h±(h = π, K) decays, are rejected by vetoing candidates with K+K−π+ (K−π+π+ and K+K−π+) invariant mass within ± 15 MeV/c2 of the D+
s (D+) meson known mass [23].
A boosted decision tree (BDT) [26] suppresses the residual background. Nine variables are input to the BDT: the decay vertex χ2 of the reconstructed B0(s) and D0 mesons; the χ2IP of the B(s)0 , D0, φ (K∗0) mesons, and of both the D0 daughters; and the pT of
the D0 and φ (K∗0) mesons. The BDT is optimised and tested using simulated signal events and events outside of the D0 mass signal region for background. Events with BDT
response larger than 0.2 are retained, resulting in a rejection of 74% of the background, while retaining 84% of the signal. The working point maximises Ns/
√
Ns+ Nb. Here, Ns
is the expected B0
s→ D0φ signal yield, computed using simulated events and assuming
that the branching fraction is equal to that of the B0→ D0K∗0 decay (as expected under
SU(3) flavour symmetry), and Nb is the background yield estimated using data events
in the sidebands outside the B0
s→ D0φ signal region (± 50 MeV/c2 around the Bs0 known
mass [23]). No multiple candidates are found for the D0φ final state. The fraction of
events with more than one candidate is 0.6% in the D0K∗0 or D0K∗0 invariant mass range of 5150–5600 MeV/c2, and the candidate retained is chosen randomly.
3
Signal yield
Signal yields are determined with an unbinned maximum likelihood fit to the D0φ and the sum of the D0K∗0 and D0K∗0 invariant mass (M ) distributions in the range 5150 <
M < 5600 MeV/c2. The two samples are fitted simultaneously with a sum of probability
density functions (PDFs) modelling signal and background contributions. The B0
s and B0 signals are described by a modified Gaussian distribution of the form
f (M ; µ, σ, αL, αR) ∝ exp −(M − µ)2 2σ2+ α L,R(M − µ)2 , (1)
where µ is the peak position, σ the width, and αL(M < µ) and αR(M > µ) parameterise
the tails. The width and the tail parameters depend on the final state, but are common to the B0
s and B0 decays. The B0 peak position and width are left free to vary in the fit
with the difference between Bs0 and B0 peak positions fixed to the current world-average value [23]. The tail parameters are fixed to values determined from simulated events and are considered among the sources of systematic uncertainty. The recently observed decay B0 → D0K+K− [24] is expected to contribute to the D0φ distribution and is modelled
] 2 ) [MeV/c φ 0 D M( 5200 5300 5400 5500 5600 ) 2 Candidates / ( 10 MeV/c 0 5 10 15
20 DataFull fit
φ 0 D → s 0 B − K + K 0 D → 0 B Comb. bkg. φ *0 D → s 0 B LHCb (a) ] 2 ) [MeV/c *0 K 0 D or *0 K 0 D M( 5200 5300 5400 5500 5600 ) 2 Candidates / ( 10 MeV/c 0 50 100
150 DataFull fit
*0 K 0 D → s 0 B *0 K 0 D → 0 B Comb. bkg. *0 K *0 D → s 0 B 0 ρ 0 D → 0 B LHCb (b)
Figure 2: Invariant mass distributions for (a) Bs0→ D0φ, and (b) B0 → D0K∗0or B0
s → D0K∗0 decays. Data points are shown in black, the total fitted PDF as solid black line, and the components as detailed in the legends.
with the same modified Gaussian distribution, but with different peak position, as that used to describe the B0
s→ D0φ decay.
Background from the B0 → D0ρ0 decay in the D0K∗0 (or D0K∗0) final state can arise
from misidentification of one of the pions from the ρ0 → π+π− decay as a kaon. The shape
of this cross-feed contribution is modelled with a Crystal Ball function [27] determined from simulated events. This background component is absent in the Bs0 → D0φ mode,
since the probability that both pions are misidentified as kaons and that their invariant mass is inside the narrow φ mass window is negligible. For similar reasons, the cross-feed between B0 → D0K∗0 and B0
s→ D0φ decays is negligible.
The decay B0 s → D
∗0K∗0, where a π0 or photon from the D∗0decay is not reconstructed,
constitutes the main background contribution to the D0K∗0 final state below the B0 mass.
Similarly, the decay Bs0 → D∗0φ is expected to contribute to the low-mass background in
the D0φ final state. These decays of a pseudoscalar to two vector mesons are modelled by
a non-parametric PDF [28] determined from simulation. The mass shape depends on the unknown fraction of longitudinal polarisation, which is assumed to be identical for the two modes and is treated as an additional free parameter in the fit.
The remaining combinatorial background is described by a linear function, with a common slope for the two considered final states, left free to vary in the fit.
Signal yield ratios are directly determined in the fit to take into account statistical correlations in the measurement of ratios of branching fractions. In total, there are 13 free parameters in the fit, including the background yields of the different components and the overall normalisation. The invariant mass distributions with the resulting fits are shown in Fig. 2.
The helicity angle distribution of the φ candidates for the Bs0 and B0 signal is investi-gated. The sPlot [29] technique is adopted to assign a weight to the events and determine the signal components, using the D0φ invariant mass as the discriminating variable. For
h
θ
cos
-1 -0.5 0 0.5 1Yield / ( 0.20 )
0 5 10 15 φ 0 D → s 0 B − K + K 0 D → 0 B simulation φ 0 D → s 0 BLHCb
Figure 3: Distribution of the cosine of the helicity angle of the φ candidates.
Table 1: Uncorrected signal yields and the peaking (charmless, S-wave) background yields.
Channel Signal Charmless background S-wave background
Bs0→ D0φ 43 ± 8 0 ± 2 2 ± 3
B0
s→ D0K
∗0 535 ± 30 4 ± 3 24 ± 7
B0→ D0K∗0 260 ± 24 4 ± 3 13 ± 6
this purpose, the requirement on cos θh > 0.4 has been lifted prior to the computation of
the signal weights. The data distributions of cos θh, shown in Fig. 3, are compared to the
expected distribution of B0
s→ D0φ decays from simulation. The distribution observed for
the B0 → D0K+K− decay candidates is consistent with the expectation that this decay is
not dominated by a pseudoscalar-vector quasi-two-body final state.
The signal yield ratios are corrected for two residual backgrounds that peak at the mass of the B0
s or B0 meson and are distributed as the signal. The first of the two backgrounds
is the charmless background due to the decays Bs0 → K+π−K∗0 and B0 → K+π−K∗0
proceeding without the presence of an intermediate D0 meson. There is no evidence of
such background in the B0
s→ D0φ channel. A large fraction of the charmless background
in the D0K∗0 final state is rejected with the requirement of a minimal D0 flight distance introduced in Sec. 2. The remaining charmless background is evaluated using candidates from the D0 sidebands. The B yields in the D0 sidebands above a linear background
are extrapolated to the D0 signal region and used to correct the signal. The uncorrected signal yields and the background contributions are given in Table 1. The other source of peaking background is due to higher mass resonances and non-resonant B0
s → D0K+K −,
] 2 ) [MeV/c − K + M(K 1000 1010 1020 1030 1040 ) 2 Yield / ( 20MeV/c 0 5 10 15 LHCb ] 2 ) [MeV/c − π + M(K 800 900 1000 ) 2 Yield / ( 10MeV/c 0 20 40 60 80 LHCb
Figure 4: Background-subtracted distributions of the reconstructed (left) φ mass from the B0s → D0φ decay and (right) K∗0 mass from the B0
s → D0K∗0 decay. The dashed red line represents the S-wave component, the solid blue line the total fit result.
Bs0 → D0K−π+, and B0 → D0K+π− decays that fall in the B0
s→ D0φ, Bs0→ D0K∗0, and
B0→ D0K∗0 signal regions, respectively. This contribution is evaluated with fits to the φ
and K∗0background-subtracted mass distributions in a wider range than the signal window. The background subtraction is performed using the sPlot technique, with the D0φ and D0K∗0 (or D0K∗0) mass as discriminating variables. A linear PDF describes the S-wave
background in the D0φ final state. A spin-one Breit-Wigner distribution convolved with a
Gaussian resolution function describes the signal, and an S-wave PDF the non-resonant background. The S-wave component in the B0→ D0K∗0 and B0
s→ D0K
∗0 channels takes
into account non-resonant and K∗0(1430) resonance contributions and uses experimental input from the LASS experiment [30]. It is approximately linear in the region of interest, ± 200 MeV/c2 around the K∗0 nominal mass. Potential interference effects between the
S-wave and the P-wave components are covered by the assigned systematic uncertainty. The φ and K∗0 mass distributions are shown in Fig. 4. The background yields, after extrapolation to the K∗0 and φ signal mass windows, are listed in Table 1.
A likelihood ratio test is employed to assess the statistical significance of the B0
s→ D0φ
signal, which is given by p2ln(Ls+b/Lb) and found to be 7.1 standard deviations. Here
Ls+b and Lb are the maximum values of the likelihoods for the signal-plus-background
and background-only hypotheses, respectively.
The ratios of branching fractions are evaluated from the uncorrected signal yields, N , and the sum of the charmless and non-resonant background yields, Nbkg, as
Rφ≡ B(B0 s→ D0φ) B(B0 s→ D0K∗0) = NBs0→D0φ NB0 s→D0K∗0 · 1 − N bkg B0s→D0φ NB0 s→D0φ 1 −N bkg B0s→D0K∗0 NB0 s→D0K∗0 · B0 s→D0K∗0 B0 s→D0φ · B(K ∗0→ K+π−) B(φ → K+K−) , (2)
and RK∗0 ≡ B(B0 s→ D0K ∗0) B(B0→ D0K∗0) = NB0 s→D0K∗0 NB0→D0K∗0 · 1 −N bkg B0s→D0K∗0 NB0 s→D0K∗0 1 −N bkg B0→D0K∗0 NB0→D0K∗0 · B0→D0K∗0 B0 s→D0K∗0 · fs fd −1 , (3)
where the ratio of the B0
s and B0 fragmentation fractions is fs/fd= 0.256 ± 0.020 [31],
the value of the φ → K+K− branching fraction is 0.489 ± 0.005 [23], and B(K∗0 → K+π−) = 2/3. The total efficiencies, , account for the geometrical acceptance of the
detector, the reconstruction, the event selection, the PID, and the trigger efficiencies. All efficiencies are computed from simulated events, except for the PID and hardware trigger efficiencies, which are obtained from data, using a high-purity calibration sample of D∗+ → D0(→ K−π+)π+ decays. The resulting ratios of branching fractions are
Rφ = 0.069 ± 0.013 and RK∗0 = 7.8 ± 0.7, where the uncertainties are statistical only.
4
Systematic uncertainties
Several sources of systematic uncertainties are considered. Those associated to the trigger and PID selection affect only Rφ and are mainly due to systematic uncertainties
in the calibration procedure. The ratios of the efficiencies of the decays B0
s → D0φ
and B0
s → D0K
∗0 for the trigger and PID are found to be 0.97 ± 0.05 and 1.08 ± 0.03,
respectively, where the errors are propagated as systematic uncertainties to Rφ.
Similarly, the uncertainty on the efficiencies of the charm meson flight distance selection affects only Rφ, where different criteria are chosen for the Bs0→ D0φ and Bs0→ D0K
∗0
modes. The ratio of the corresponding efficiencies is found to be 1.27 ± 0.03, where the uncertainty includes a contribution from the difference between data and simulation. In order to estimate the efficiency in data, the fit to the invariant mass of the B candidates is performed to data samples selected with all criteria except that on the flight distance. For this sample, the charmless background contribution is estimated using events in the upper D mass sideband and subtracted from the signal yields.
The ratio of the efficiencies for the decays Bs0→ D0φ and B0
s→ D0K∗0 of the remaining
selection criteria is found to be 1.21 ± 0.03, where the deviation from unity is mainly due to the different widths and mass windows for the φ and K∗0 resonances. The ratio of the efficiencies for the decays Bs0→ D0K∗0 and B0→ D0K∗0 is found from simulation
to be 1.04 ± 0.01. The uncertainties on these efficiencies are propagated as systematic uncertainties due to the selection.
The fit procedure is validated using simulated pseudo-experiments. The fit bias, relative to the fitted ratio, is evaluated to be 1.4% for Rφ and 0.2% for RK∗0 and is assigned as systematic uncertainty. The signal model uncertainty is evaluated by varying the fixed signal parameters by 10%, which is about three times the difference between data and simulation, as determined by a fit where those parameters are free to vary. The background
Table 2: Absolute systematic uncertainties of the measured ratio of branching fractions. The total is obtained as sum in quadrature of the different contributions.
Source Rφ RK∗0 Trigger 0.003 -PID 0.002 -Flight distance 0.002 -Selection 0.002 -Simulation statistics 0.001 0.10 Fit bias 0.001 0.03 Signal model 0.001 0.04 Background model 0.001 0.01 Charmless correction 0.003 0.10 Non-resonant correction 0.004 0.22 φ branching fraction 0.001 -Total 0.007 0.26
shape uncertainty is determined from the bias in the results obtained by fitting samples generated with an alternative (exponential) combinatorial background model.
The uncertainties on the charmless background yields given in Table 1 are assumed to be uncorrelated and are propagated to assign the associated systematic uncertainty. Similarly, the statistical uncertainties on the S-wave background yields are propagated to Rφ and RK∗0 to assign respective systematic uncertainties due to the non-resonant correction.
A summary of the systematic uncertainties is given in Table 2. The uncertainty on the fragmentation fraction fs/fd, which is the dominant systematic uncertainty for RK∗0, is not included, and is listed separately.
5
Results and conclusions
The significance of the B0
s→ D0φ signal, including systematic uncertainties, is obtained by
scaling the statistical significance with the ratio of the statistical to the total (statistical and systematic) uncertainty on the signal yield. It is found to be 6.5 standard deviations. This decay is therefore observed for the first time.
The ratios of branching fractions are found to be
Rφ= 0.069 ± 0.013 (stat) ± 0.007 (syst),
RK∗0 = 7.8 ± 0.7 (stat) ± 0.3 (syst) ± 0.6 (fs/fd).
Bs0→ D0K∗0 branching fraction is calculated to be
B(B0
s→ D0K ∗0
) = [3.3±0.3 (stat)±0.1 (syst)±0.3 (fs/fd)±0.5 (B(B0→ D0K∗0))]×10−4.
This result is consistent with and improves on the previous determination by LHCb [12], which is based on an independent data sample. Using the above results for Rφ, RK∗0 and the B0→ D0K∗0 branching fraction, the branching fraction for B0
s→ D0φ is calculated to be B(B0 s→ D 0φ) = [2.3 ± 0.4 (stat) ± 0.2 (syst) ± 0.2 (f s/fd) ± 0.3 (B(B0→ D0K∗0))] × 10−5,
which takes into account the correlation in the statistical uncertainties between Rφ and
RK∗0 of −13.6%. The correlation between the corresponding systematic uncertainties is negligible. The central value is about a factor two smaller than the branching fraction for the B0→ D0K∗0 decay and supports the observation of SU(3) breaking effects in other
colour suppressed B0
(s) → D0V decays [12], where V is a vector meson. With larger data
samples, the Bs0→ D0φ decay will contribute to the measurements of the CP violating
phases γ and βs.
Acknowledgements
We express our gratitude to our colleagues in the CERN accelerator departments for the excellent performance of the LHC. We thank the technical and administrative staff at the LHCb institutes. We acknowledge support from CERN and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); NSFC (China); CNRS/IN2P3 and Region Auvergne (France); BMBF, DFG, HGF and MPG (Germany); SFI (Ireland); INFN (Italy); FOM and NWO (The Netherlands); SCSR (Poland); MEN/IFA (Romania); MinES, Rosatom, RFBR and NRC “Kurchatov Institute” (Russia); MinECo, XuntaGal and GENCAT (Spain); SNSF and SER (Switzerland); NAS Ukraine (Ukraine); STFC (United Kingdom); NSF (USA). We also acknowledge the support received from the ERC under FP7. The Tier1 computing centres are supported by IN2P3 (France), KIT and BMBF (Germany), INFN (Italy), NWO and SURF (The Netherlands), PIC (Spain), GridPP (United Kingdom). We are thankful for the computing resources put at our disposal by Yandex LLC (Russia), as well as to the communities behind the multiple open source software packages that we depend on.
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